One possible way to spot them in principle is to use the fact that they act as powerful lenses. Any light passing near the black hole gets bent and any stars that we see behind the black hole get distorted. This is a computer generated image of the effect
a black hole would have if it passed in front of a field of stars. Unfortunately,
this is only useful if the black hole is moving relative to the distant star field, so that
we can detect the change. The black hole has to be passing by fairly close to the Earth, and we have to be looking at the right place at the right time.